A Treatise On The Principles And Practice Of Harbour Engineering

Ô2 HARBOUR ENGINEERING. So far as soundness and strength are concerned, there are few trees which are incapable of supplying logs and balks of a thoroughly satisfactory character. In harbour work, however, durability is the crucial considération and the conditions attaching to that qualification are inuch more exacting than those which govern the choice of suitable timbers for constructive purposes elsewhere. The alternations of exposure to the atmosphère and submersion in the sea, due to tidal fluctuations, constitute in themselves a most fertile source of decomposition, such as is not experienced in any other environment nor associated with any other branch of engineering. And, as if this were not sufficient, there is allied therewith a most pernicious and deadly subjection to the mechauical attacks of insectile 1 borers, which infest the waters of most ports. In addition to the question of durability, however, there are the subsidiary, but no less essential, considérations of available scantling, cost, and facility of supply, eaeh of which demands the careful attention of the engineer. It is not proposed to enter into any lengthy dissertation of a botanical nature on the very great variety of trees which are available for engineering purposes; it will be sufficient to confine our attention to details of a practical kind in connection with those comparatively few species which have obtained wide and general recognition in connection with maritime work. These may be enumerated briefly in three groups :— I. Greenheart, Mora, and the Eucalypti. These woods are extremely durable and highly repellent of insects. II. Teak and Oak. These are also very durable, but subject to insect attack. III. Beech, Elm, and Pine. These are moderately durable, and they succumb easily to insects. By far the most important group to the harbour engineer is the first. To this we must pay greatest attention, leaving the other groups, though they comprise timbers of more extensive use, to be but briefly noticed. Greenheart (Nectandra Ilodiæi) is an American product, the tree being a native of Guiana and the adjacent states of the South American continent, where it grows very abundantly in tracts lying within a hundred miles of the coast-line. It is a wood of extreme hardness and durability, with a very fine and compact, though uneven, grain. Its resistance to crushing is enormous, but it is very brittle and it splits under the least provocation. Before sawing, logs have to be bound very tightly with chains and wedges on each side of the projected cut; otherwise there is great danger of splitting, and a crack once started is prevented from extending with difficulty greater than that which characterises avoidance of the danger in the first instance. Greenheart contains a poisonous oil, which renders necessary considérable circumspection on the part of carpenters and others engaged in dressing it. 1 Objection may be taken to the use of this word in this connection. It is difficult, however, to find an accurate generic narne for these pests. The term insect is applied under lieense which is justifiable, since no confusion is likely to arise from its use.